1. Field of the Invention
The present invention generally relates to an excimer laser beam irradiation apparatus used for optically machining or working or processing (e.g. etching) a workpiece such as, for example, a multi-layer printed substrate to thereby form apertures, holes exemplified by so-called via-holes, through-holes and the like by illuminating or irradiating the substrate with an excimer laser beam through a patterning mask having a pattern to be formed in the workpiece. More particularly, the present invention is concerned with an excimer laser beam irradiation apparatus which can allows workpieces to be processed in a stable state with a uniformized intensity distribution of the excimer laser beam.
2. Description of Related Art
For a better understanding of the concept underlying the present invention, description will first be made in some detail of a hitherto known excimer laser beam irradiation apparatus employed in the conventional optical processing or machining equipment of the above-mentioned type by reference to FIGS. 14 and 16 of the drawings, in which FIG. 14 is a perspective view showing only schematically a typical one of the optical processing apparatuses known heretofore. For more particular of this known apparatus, reference should be made to "Collection of Theses in 28-th Convention of Laser Processing Engineers of Japan (28-th LASER NETSUKAKO KENKYUKAI RONBUNSHU)", pp. 51-58, (July, 1992).
Referring to FIG. 14, the optical processing apparatus illustrated therein includes a light source system constituted by an excimer laser oscillator 1 for generating an excimer laser beam L0 having a rectangular shape in cross section. Disposed at positions downstream of an output port of the laser oscillator 1 along an optical path of the excimer laser beam L0 are three mirrors 3a, 3b and 3c which cooperate to constitute a beam path adjusting system for adjusting a beam direction and a beam rotation angle of the excimer laser beam L0 emitted from the excimer laser oscillator 11.
On the other hand, disposed in succession to the beam path adjusting system mentioned above along the optical path of the excimer laser beam L0 is a beam shaping optical system which is composed of two sets of concave and convex cylindrical lenses 4a, 4b; 4c, 4d for converting or shaping the excimer laser beam L0 of rectangular cross-section into a sheet-like or flat excimer laser beam L1, wherein the convex lenses 4a and 4c are disposed in opposition to the concave lenses 4b and 4d, respectively. These lens elements are fixedly mounted on a stationary support 5. The excimer laser beam L1 leaving the beam shaping optical system (4a, 4b; 4c, 4d) is reflected by an incident-angle adjusting mirror 7 disposed on the optical path of the excimer laser beam L1.
A patterning mask 8 onto which the excimer laser beam L1 impinges, as projected by the incident-angle adjusting mirror 7, is composed of a light-transmissive base plate or substrate 8a which allows the excimer laser beam L1 to transmit therethrough. Deposited over the light-transmissive substrate 8a are a reflecting layer 8b for reflecting the excimer laser beam L1, wherein through-holes or apertures 8c allowing the excimer laser beam L1 to pass therethrough are formed in the reflecting layer 8b. Needless to say, these through-holes or apertures 8c form a pattern to be imaged or transferred to a workpiece, as will hereinafter be described in more detail.
Disposed in association with the patterning mask 8 is a mask moving mechanism 9 provided for moving the patterning mask 8 in directions orthogonal to the optical axis, i.e., in the x- and y-directions to thereby cause the excimer laser beam L1 to scan the top surface of the patterning mask 8 in the relative sense.
Disposed above and in opposition to the patterning mask 8 is a high reflectivity mirror 10 which serves as a reflecting means for redirecting the excimer laser beam L1 reflected at the reflecting layer 8b toward the patterning mask 8 repetitively, as described later on.
On the other hand, an imaging lens 11 is disposed underneath the patterning mask 8 at a position on the optical path of the excimer laser beam L2 which leaves the patterning mask 8. A workpiece 12 to be optically processed (or optically machined) is illuminated or irradiated with the excimer laser beam L2 having passed through the imaging lens 11 whose function is to transfer the aforementioned pattern formed in the patterning mask 8 onto the workpiece 12 in the form of an inverted image.
A workpiece mounting platform 13 is disposed beneath the imaging lens 11 for mounting and positioning the workpiece 12. On the other hand, the workpiece mounting platform 13 is supported on a workpiece moving mechanism 14 which is adapted to be moved in directions orthogonal to the optical axis of the imaging lens 11, i.e., in the x- and y-directions, respectively. The workpiece moving mechanism 14 in turn is mounted on a vibration isolating common bed 15.
Operations of both the mask moving mechanism 9 and the workpiece moving mechanism 14 are controlled with high accuracy by a control unit 16 which may be constituted by a microcomputer and which is also in charge of controlling the excimer laser oscillator 1. Additionally, provided is a processing monitor system 17 which is disposed above the workpiece 12 for the purpose of inspecting the positions and geometrical factors of the pattern formed in the workpiece 12.
Description will now be made by reference to FIGS. 15A and 15B, in which FIG. 15A is a side elevational view showing schematically and exaggeratedly an optical system including the patterning mask 8, the imaging lens 11 and associated parts, and FIG. 15B is a view for graphically illustrating an intensity distribution of the excimer laser beam L2 on the workpiece 12 as viewed along the y-direction. At this juncture, it is presumed that the excimer laser beam L1 undergone multiple reflections between the patterning mask 8 an the high reflectivity mirror 10 is shifted in the y-axis direction.
Referring to FIG. 15A, the light-transmissive base plate or substrate 8a of the patterning mask 8 is formed of a synthetic quartz material and allows the excimer laser beam L1 leaving the lens system (4a-4d) and reflected at the incident angle adjusting mirror 7 to pass therethrough by way of the light passing holes or apertures 8c. As a result of the irradiation with the excimer laser beam L2 passed through the masks 8 and the imaging lens 11, holes such as the via-holes 18 are formed in the workpieces 12 in a pattern corresponding to that of the holes 8c of the patterning mask 8. The reflecting layer 8b is deposited over the light-transmissive substrate 8a through a vapor deposition process with the hollow holes 8c being left in a predetermined pattern to be transferred to the workpiece 12. On the other hand, the reflecting layer 8b is formed in the form of a film such as an aluminum film, a multi-layer dielectric film or the like which exhibits a high reflectivity (e.g., reflectivity not less than 99%). As mentioned above, the reflecting layer 8b is pierced with a large number of hollow holes 8c each having a diameter, for example, on the order of 20 .mu.m.
The imaging lens 11 is implemented as a high performance lens having aberrations suppressed to a possible minimum over a region of a large field angle for imaging the pattern formed in the patterning mask 8, i.e., pattern of the hollow holes 8c onto the workpiece 12 with high accuracy.
Parenthetically, FIG. 16A is a side view showing schematically and exaggeratedly the processing optical system and associated parts, as viewed in the y-axis direction, and FIG. 16B is a view for illustrating intensity distribution of the excimer laser beam L2 on the workpiece 12, as viewed in the x-axis direction.
Next, referring to FIGS. 14 to 16, description will be made of operation of the excimer laser beam irradiation apparatus of the structure elucidated above.
First, referring to FIG. 15A, a part of light rays of the excimer laser beam L1 incident on the patterning mask 8 at a lateral side thereof (at a right-hand side as viewed in the figure) obliquely from the above transmits through the hollow holes 8c to form the excimer laser beam L2 which contributes to the optical processing or etching.
The other part of the light rays of the excimer laser beam L1 incident on the patterning mask 8 is reflected by the reflecting layer 8b toward the high-reflectivity mirror 10 which reflects back the incident light rays again onto the patterning mask 8.
As can be seen in FIG. 15A, a part of the excimer laser beam L1 reflected toward the patterning mask 8 by the high-reflectivity mirror 10 is caused to shift progressively in the y-direction (i.e., from the right to the left, as viewed in FIG. 15A) due to reflections between the patterning mask 8 and the workpiece 10 and incidence of the excimer laser beam L1 with an incident angle .theta. smaller than 90.degree. relative to the vertical. More specifically, the position at which the excimer laser beam L1 impinges on the patterning mask 8 at a second time after reflection at the high-reflectivity mirror 10 is deviated in the y-direction (i.e., to the left, as viewed in FIG. 15A) from the position at which the excimer laser beam L1 impinges on the patterning mask 8 at the first time. Such reflection and shift of the excimer laser beam L1 is repeated until the excimer laser beam L1 leaves the cavity defined between the patterning mask 8 and the high-reflectivity mirror 10, although some part of the excimer laser beam L1 is allowed to pass through the patterning mask 8 via the pattern of hollow holes 8c.
Owing to the repetitive or multiple reflections of the excimer laser beam L1 between the patterning mask 8 and the high reflectivity mirror 10 and the shifts in the y-direction, the pattern holes 8c of the patterning mask 8 is transferred to the workpiece 12 by way of the imaging lens 11. In that case, it is naturally required to maintain the intensity of the excimer laser beam L1 substantially at a predetermined constant level, being protected against attenuation. Of course, the excimer laser beam L2 transmitted through the pattern holes 8c of the patterning mask 8 is focused onto the workpiece 12 via the imaging lens 11. As a result of this, apertures or holes such as the via-holes 18 are formed in the workpiece 12 in a pattern corresponding to an inverted image of the pattern of the holes 8c formed in the patterning mask 8.
In practical applications, it is naturally noted that there arises a possibility of the intensity of the excimer laser beam L1 becomes gradually lower as the excimer laser beam L1 moves from one end of the high reflectivity mirror 10 to the other end in the y-axis direction while being reflected between the patterning mask 8 and the high reflectivity mirror 10, as can be seen in FIG. 15B. As a consequence of this, the intensity of the excimer laser beam L2 impinging onto the workpiece 12 gradually decreases in the course of the reflections and the positional shifts in dependence on the positions along the y-axis in the opposite direction (i.e., in a minus (-) y-axis direction) because the pattern of the patterning mask 8 imaged onto the workpiece 12 is inverted.
On the other hand, when viewed in the x-axis direction (see FIG. 16A), the excimer laser beam L1 incident at a center portion of the high reflectivity mirror 10 undergoes successive reflections between the patterning mask 8 and the high reflectivity mirror 10 in the directions toward both ends, as a result of which the intensity distribution of the excimer laser beam L2 in the x-axis direction will assume such a profile as illustrated in FIG. 16B.
In conjunction with the imaging lens 11, it is further noted that the imaging lens 11 is realized by a high-performance lens system whose aberrations are suppressed to a possible minimum over a major region of the image plane, as mentioned previously, in order to optically transfer or image the pattern of the patterning mask 8 onto the workpiece 12 with high accuracy. Thus, in the case where the workpiece 12 is, for example, a multi-layer printed substrate, the size thereof is usually on the order of 100 mm.times.100 mm. Accordingly, when this area is to be optically processed in a single step, the lens system of extremely expensiveness has to be employed as the imaging lens 11.
Such being the circumstances, in the optical processing apparatus known heretofore, the optical processing over a large area of the workpiece 12 is realized by moving both the patterning mask 8 and the workpiece 12 in synchronism by using the mask moving mechanism 9 and the workpiece moving mechanism 14 to thereby effectively scan the workpiece 12 with the positionally stationary excimer laser beam L2 in the relative sense.
By way of example, when the magnification of the imaging lens 11 is "1/2", the patterning mask 8 is scanned with the excimer laser beam L1 at a speed v in the x-axis direction while the workpiece 12 on which the inverted image of the hole pattern of the patterning mask 8 is to be copied is simultaneously scanned in the opposite direction (i.e., minus (-) x-direction) at a speed of v/2. Upon completion of the scanning in the x-direction, the workpiece 12 is displaced stepwise in the y-direction to perform again the scanning operation mentioned above. By repeating the scanning operation in this way, the whole surface of the workpiece 12 can optically be processed.
Subsequently, upon completion of the scanning displacement of the patterning mask 8 and the workpiece 12 in the x-axis direction, the patterning mask 8 and the workpiece 12 are fed stepwise by one increment in the y-axis direction, whereupon the scanning displacement mentioned above is sequentially repeated. Thus, the whole surface of the workpiece 12 is illuminated or irradiated with the excimer laser beam.
In this conjunction, it is however noted that the pattern which is not uniformized (i.e., not constant) in respect to the intensity distribution, as shown in FIG. 15B, is imaged onto the workpiece 12.
As will now be understood from the above description, in the case of the hitherto known excimer laser beam irradiation apparatus for the optical processing, the patterning mask 8 and the workpiece 12 are moved for scanning the latter with the excimer laser beam L2 in the direction (x-axis direction) orthogonal to the direction (y-axis direction) in which the excimer laser beam L1 shifts while undergoing multiple reflections between the patterning mask 8 and the high reflectivity mirror 10. Consequently, the intensity distribution of the excimer laser beam L1 undergoing the multiple reflections between the patterning mask 8 and the high reflectivity mirror 10 can not always be maintained to be essentially constant or uniform. Thus, there may arise such a situation that the intensity distribution of laser beam irradiation can not be realized uniformly over the workpiece 12. Under the circumstances, the processed state of the workpiece 12 may become non-uniform although it depends on the material of the workpiece 12, the processing or working precision attainable with the optical system and other factors as well, giving rise to a serious problem.
Moreover, in the conventional excimer laser beam irradiation apparatus, variation in the thickness and the material of the workpiece 12 as well as variation in the speed at which the patterning mask 8 and the workpiece 12 are moved for the scanning operation provide obstacles to realization of the uniform processing of the workpiece 12 to a disadvantage.